(1) Field of the Invention
The present invention relates to a motion picture display device for sequentially displaying, without interruption, different types of motion pictures having different picture configurations. The present invention especially relates to a picture memory management technique.
(2) Description of the Related Art
For displaying a motion picture having been decoded, image processing may be required to, for example, scale up the picture size to an appropriate size. Conventionally, a picture memory is employed to temporarily store picture data resulting from such image processing.
A motion picture display device controls writing of picture data to and reading of the picture data from the picture memory. The motion picture display device displays a motion picture by sequentially reading picture data from the picture memory in accordance with the timing determined by the NTSC or PAL format.
Taking the NTSC format as an example, a frame of picture data is composed of a top field and bottom field. The motion picture display device alternately writes the top field data and the bottom field data into different areas of the picture memory at every field period, i.e., half a frame period. In parallel with the data write, the motion picture display device reads picture data from the picture memory at every field period, so that the field data already written to the picture memory is supplied to display the motion picture.
More specifically, the motion picture display device starts a write operation to write, for example, the bottom field data of a frame in one field period. In parallel with the write operation and within the same field period, the motion picture display device performs a read operation to display the top field data of the same frame having been written in the previous field period. In the subsequent field period, the motion picture display device starts a read operation to display the bottom field data written in the previous field period, in parallel with a write operation to write the top field data of a subsequent frame to be displayed in a yet another field period subsequent to the current field period.
It should be noted that picture data written to the picture memory within one field period is worth one field, whereas picture data read from the picture memory within one field period may be worth one frame, rather than one field. This is because a data read operation to display either of the top and bottom fields requires data of the other field.
It is not necessary to synchronize read and write operations to and from the picture memory. Instead, a read operation from and a write operation to the picture memory may be controlled to be out of synchronism, which is called asynchronous control. Under asynchronous control, read operations and write operations are separately controlled and there is a delay of one field period between a write operation and a read operation of data of a specific field.
Unfortunately, however, the asynchronous access control involves a risk of so-called overtaking, although the risk is small. Overtaking refers to a phenomenon in which a write operation of picture data to the picture memory occurs before previously written picture data is read from the memory or a read operation to read picture data from the picture memory occurs before the picture data is written. An occurrence of overtaking may cause image disturbance for one field period.
The overtaking phenomenon occurs as a result of making every access to the picture memory at the base address, irrespective of the fact that motion pictures of different aspect ratios requires memory access at different access start timing.
In order to prevent an occurrence of overtaking, the following patent document suggests providing two picture memories.
[Patent Document 1] Patent Application Publication No. JP 06-326921
Unfortunately, however, sequential display of motion pictures having different aspect ratios and/or display plane positions will inevitably cause image disturbance at the time of a changeover between the motion pictures.
For the purpose of more specific description, it is supposed that a motion picture display device is designed for displaying, as a standard, a motion picture having an aspect ratio of 4:3. It is also supposed that the motion picture display device sequentially displays a motion picture having an aspect ratio of 16:9 and then a motion picture having an aspect ratio of 4:3 under asynchronous-access control.
The motion picture display device starts reading the picture memory in accordance with the display timing of a motion picture to be displayed. Thus, in the case of displaying the 16:9 motion picture, no picture data is read from the picture memory during the time output is made to display black bars at the top and bottom of the display plane. As a consequence, the timing of a read operation from the picture memory for displaying the 16:9 motion picture is later than the timing of a read operation from the picture memory for displaying the 4:3 motion picture. The delay time corresponds to the time taken to output the black bar. Similarly, since a write operation to the picture memory starts one field period before the start of a subsequent read operation as described-above, the timing of a write operation to the picture memory for displaying the 16:9 motion picture is later than the timing of a write operation for displaying the 4:3 motion picture.
During the time the motion picture display device is reading the last field data of the 16:9 motion picture from the picture memory, the first field data of the 4:3 motion picture, which is to be displayed one field period later, is written to the picture memory. Normally, the last field data of the 16:9 motion picture needs to be read from the picture memory before the first field of the 4:3 motion picture is written to the picture memory.
For the reason described above, however, the start timing of a read operation for the 16:9 motion picture is later than the display timing of the 4:3 motion picture. Consequently, the first write operation for the 4:3 motion picture overtakes the last read operation for the 16:9 motion picture. Thus, the picture data read by the read operation is not the picture data of the 16:9 motion picture written to the picture memory in the immediately previous field period. Rather, the picture data of the 4:3 motion picture written to the picture data as a result of the overtaking is read. This inevitably causes the image disturbance.
Although the patent document 1 cited above manages to solve the problem, the disclosed technique requires two picture memories to be provided, which increases the manufacturing cost of the device.